Muscles are composed of a variety of muscle fibers with different biochemical, structural and physiological properties. The characteristics of these fibers can change in response to a number of stimuli, such as physical and neuronal activity, hormonal changes, muscle damage and muscle diseases. The underlying mechanisms of this muscle plasticity are largely unknown, but are important for understanding processes involved in muscle diseases and the effects of exercise on degeneration and regeneration. The purpose of this study is to investigate the molecular mechanisms underlying muscle plasticity due to exercise. The animal model used is the spinal cord transected paralyzed/exercised rat. Soleus and extensor digitorum longus muscles from spinal transected rats (T10 transection) will be studied at time points shortly (6 hr - 5 days) after the onset of exercise to address the hypothesis that muscle specific transcription factors and satellite cells are involved in muscle plasticity. Specifically, it will be determined if satellite cells are activated and if activation is correlated with muscle damage.